The present invention relates to a semiconductor device incorporating a lead frame. More particularly, the invention relates to techniques adapted advantageously to a semiconductor device with a lead frame comprising numerous leads as well as heat radiation plates.
LSIs and other semiconductor devices have known ever-higher levels of circuit integration while incorporating higher functions and more complicated circuits than ever before. The enhanced functionality requires furnishing each semiconductor device with a large number of external terminals. This in turns involves increasing the number of pad electrodes provided on a semiconductor chip as well as the number of leads, i.e., external terminals of the semiconductor device. A typical logic semiconductor device may have hundreds of external terminals. Semiconductor devices of the so-called QFP (quad flat package) type, well known as a family of semiconductor devices each having numerous external terminals, are generally mounted on one side of a substrate and called surface-mounted semiconductor devices. The QFP type semiconductor device is suitable for accommodating a large number of leads because each of four sides of the package enclosing the semiconductor chip carry a plurality of leads When mounted on a substrate, this type of semiconductor device permits an effective use of space around it.
A lead frame used in the assembling of such QFP type semiconductors is discussed illustratively in xe2x80x9cVLSI Packaging Techniques (Vol. 1)xe2x80x9d published by Nikkei BP (in Japan) on May 31, 1993, pp. 155-164. In particular, specific patterns of the frame are shown on pp. 157 and 159.
Fine structures of the semiconductor chip comprise an increasing number of elements each operating at a higher speed than ever. This causes an increase of heat generation from the semiconductor chip. The problem is avoided illustratively by a semiconductor device having a heat spreader, as described in the xe2x80x9cVLSI Packaging Techniquesxe2x80x9d (Vol. 2), pp. 200-203. The semiconductor device has its semiconductor chip furnished with a heat spreader arrangement to promote heat dissipation of the device.
In accommodating a large number of leads, the lead frame needs to have its lead-to-lead spacing (i.e., lead pitch) narrowed and the width of its leads lessened.
The semiconductor chip also comprises numerous pad electrodes whose presence is necessitated by the enhanced functionality of the semiconductor device. Meanwhile, the spacing between pad electrodes (i.e., pad pitch) has been reduced over the years. Whereas there are different pad pitches for different semiconductor chips in general, the need to obtain as many chips as possible per wafer involves establishing the smallest possible chip size. The trend in turn requires having the smallest possible pitch between pad electrodes.
Given such reduced pad pitches and under restrictions associated therewith, the process of bonding the many leads to the corresponding pad electrodes using wires made of gold or like material tends to trigger an increasing number of short-circuits between adjacent wires.
During resin molding after the wire bonding, a decline in the mechanical strengths of leads or a narrowed wire spacing may let wires be deformed by molding resin fluidity. The deformation called wire flow can result in short-circuited wires.
Furthermore, in a QFP, an area in which to lay out leads becomes narrower the closer it gets to a centrally located semiconductor chip. The thickness and the pitch of the leads are subject to limitations stemming from the limitation of the manufacture precision of the lead. More specifically, lead pitch cannot be made sufficiently fine compared with pad patch on the semiconductor chip. As the semiconductor chip shrinks in external dimensions, it becomes increasingly difficult to bring the tips of the leads close to the chip. When the lead tips to be bonded are distanced from the pad electrodes of the semiconductor chip under such circumstances, wires for bonding the pads to the leads must be extended. Extended wires are likely to cause more short-circuits or result in more wire flow than before.
While today""s practical pad pitches are down to about 80 xcexcm, the required pitch is expected to reach 60 to 45 xcexcm in the future. As chips shrink further, bonding wires are extended correspondingly. At present, it is necessary to keep the wire length to a maximum of 5 or 6 mm in order to ensure stable bonding. This requires further reducing the pitch of lead tips so as to avert wire extensions.
FIG. 1 shows results of simulations performed by the inventors about wire bonding. On 256-pin semiconductor chips with different pad pitches, correlations were simulated between inner lead tip pitches on the one hand and wire lengths for stable bonding on the other hand. The simulations revealed the need to restrict the lead tip pitch to a maximum of 180 xcexcm with respect to the 60 xcexcm pad pitch in order to ensure stable bonding.
Such micro-fabrication of the leads is bound to lower their mechanical strength. Even an extremely limited amount of force can thus deform the tenuous lead formation. The deformed leads trigger short-circuits.
A conventional solution to the above problem is the fastening of inner leads using an insulating tape to prevent lead deformation. FIG. 2 is a plan view of a conventionally structured tape-fastened lead frame. FIG. 3 is a cross-sectional view of a resin-sealed semiconductor device fabricated by use of the lead frame in FIG. 2.
The lead frame is illustratively made of a copper alloy. A semiconductor chip 1 (indicated by broken lines) is fixed to a tab 2. A plurality of leads 3 are located around the entire periphery of the mounted semiconductor chip 1. The leads 3 come in two types: inner leads 4 and outer leads 5. The tips of the inner leads 4 surround the semiconductor chip 1.
The leads 3 are integrated with a dam bar 6 or with a tie bar 8 constituting a framework of the lead frame. The inner and outer leads 4 and 5 are formed inside and outside of the dam bar 6 respectively. The tab 2 is supported by tab suspending leads 7 furnished across the inner leads 4. The inner leads 4 and the tab suspending leads 7 are fastened to a rectangular insulating tape 9.
In the case of a semiconductor device using the above-described lead frame, the semiconductor chip 1 is fixed to the tab 2 by resin or by silver paste while the inner leads 4 are connected to pad electrodes 10 of the chip 1 by bonding wires 11. After bonding, the semiconductor chip 1, tab 3, inner leads 4 and bonding wires 11 are molded by a molding member 12 illustratively made of epoxy resin. The dam bar 6 and tie bar 8 are cut so that the leads 3 are electrically isolated from one another. Thereafter, the outer leads 5 extending from the molding member 12 are illustratively formed in gull wing fashion as shown in FIG. 3. This completes fabrication of the semiconductor device.
With the tape-fastened lead frame, as shown in FIGS. 2 and 3, a middle part of the inner leads 4 is secured by the tape 9 to allow for flexible uses of the frame. In other words, the tape 9 is positioned away from the tips of the inner leads 4. This is an inefficient and unstable structure for fastening the inner lead tips to which wires are to be bonded.
Furthermore, some recently developed semiconductor devices have been subject to significant heat generation from semiconductor chips because of their enhanced functionality and high performance. These devices have their semiconductor chips equipped with a heat radiation plate such as a heat spreader to facilitate heat dissipation.
FIG. 4 is a plan view of a lead frame for use with a heat spreader-incorporating QFP (called HQFP hereunder), wherein a copper foil devised by the inventors is attached by adhesive to a semiconductor chip as a heat radiation plate. This setup has not been disclosed until now. FIG. 5 is a cross-sectional view of a semiconductor device fabricated by use of the lead frame in FIG. 4.
As opposed to the lead frame and semiconductor device discussed earlier, this semiconductor chip 1 (indicated by broken lines) is fastened to a heat radiation plate 13. The inner leads 4 are also fixed to the heat radiation plate 13.
Such HQFP type semiconductor devices comprise numerous contacts between the molding member 12 and the heat radiation plate 13. Because of a feeble adhesive strength between resin (i.e., molding member 12) and metal (heat radiation plate 13), moisture absorbed in the interface between the molding member 12 and the heat radiation plate 13 can evaporate and expand during reflow heating, causing a crack in the package. The reflow heating occurs during the process of mounting a surface-mounted semiconductor device onto a substrate. The semiconductor device of FIG. 4 addresses the reflow problem by having a round hole provided in the middle of the heat radiation plate 13, the hole allowing the molding member 12 to contact the semiconductor chip 1. However, this structure is still not sufficient to overcome the problem.
It is therefore an object of the present invention to provide techniques for stabilizing the bonding of a semiconductor device having numerous leads.
It is another object of the present invention to provide techniques for preventing a crack in the package of a semiconductor device furnished with a heat radiation plate.
A summary of typical ones of the invention disclosed in the present application will be described in brief in the following manner.
In a semiconductor device having a semiconductor chip fixed to a radiation plate and sealed by a sealant, the semiconductor chip being connected to tips of inner leads having a lead width of w and a lead thickness of t, wherein the lead width w is less than the lead thickness t (w less than t), and wherein at least the tips of the inner leads are fastened to the radiation plate.
In a semiconductor device having a semiconductor chip fixed to a radiation plate and sealed by a sealant, the semiconductor chip being connected to tips of inner leads having a lead width of w and a lead thickness of t, wherein the lead width w is less than the lead thickness t and wherein at least the tips of the inner leads are fastened to the radiation plate so as to support the radiation plate. This structure eliminates the need for installing conventional suspending leads supporting the radiation plate.
In a semiconductor device having a semiconductor chip fixed to a radiation plate and sealed by a sealant, the semiconductor chip being connected to tips of inner leads having a lead pitch of p, a lead width of w and a lead thickness of t, wherein the lead width w is less than the lead thickness (w less than t), wherein the lead pitch p is equal to or less than 1.2 times the lead thickness t (pxe2x89xa61.2 t), and wherein at least the tips of the inner leads are fastened to the radiation plate.
In a semiconductor device having a semiconductor chip fixed to a radiation plate and sealed by a sealant, the radiation plate having slits made therein in a radial fashion forming heat propagation paths radiating from a semiconductor chip mounting area on the plate toward inner leads.
In a semiconductor device having a semiconductor chip fixed to a radiation plate and sealed by a sealant, wherein tips of inner leads (tip thickness txe2x80x2) are made thinner than the other portions of the inner leads (lead thickness t), and wherein at least the tips of the inner leads are fastened to the radiation plate.
In a method for fabricating a semiconductor device having a semiconductor chip fixed to a radiation plate and sealed by a sealant, the method comprising the steps of: connecting the semiconductor chip to tips of inner leads having a lead pitch of p, a lead width of w and a lead thickness of t, wherein the lead width w is less than the lead thickness t (w less than t) and wherein the lead pitch p is equal to or less than 1.2 times the lead thickness t (pxe2x89xa61.2 t); fastening at least the tips of the inner leads to the radiation plate; and connecting the inner leads to pad electrodes of the semiconductor chip.
In a method for fabricating a semiconductor device having a semiconductor chip fixed to a radiation plate and sealed by a sealant, the method comprising the steps of: making slits in the radiation plate in a radial fashion forming heat propagation paths radiating from a semiconductor chip mounting area on the plate toward inner leads; and forming the sealant by letting it penetrate through the slits in the radiation plate during resin sealing.
In a method for fabricating a semiconductor device having a semiconductor chip fixed to a radiation plate and sealed by a sealant, the method comprising the steps of: fastening the semiconductor chip to the radiation plate on a lead frame wherein tips of inner leads (tip thickness txe2x80x2) are made thinner than the other portions of the inner leads (lead thickness t); and connecting the inner leads to pad electrodes of the semiconductor chip wherein at least the tips of the inner leads are fastened to the radiation plate.
In a lead frame comprising a plurality of leads and a radiation plate having a semiconductor chip mounting area onto which to fix a semiconductor chip, the semiconductor chip being connected to tips of inner leads among the leads, the inner lead tips having a lead pitch of p, a lead width of w and a lead thickness of t, wherein the lead width w is less than the lead thickness t (w less than t), wherein the lead pitch p is equal to or less than 1.2 times the lead thickness t (pxe2x89xa61.2 t), and wherein at least the tips of the inner leads are fastened to the radiation plate.
In a lead frame comprising a plurality of leads and a radiation plate having a semiconductor chip mounting area onto which to fix a semiconductor chip, the radiation plate having slits made therein in a radial fashion forming heat propagation paths radiating from the semiconductor chip mounting area on the plate toward inner leads.
In a lead frame comprising a plurality of leads and a radiation plate having a semiconductor chip mounting area onto which to fix a semiconductor chip, wherein tips of inner leads among the leads (tip thickness txe2x80x2) are made thinner than the other portions of the inner leads (lead thickness t), and wherein at least the tips of the inner leads are fastened to the radiation plate.
According to the present invention, there is provided a semiconductor device comprising: a heat radiation plate including a main surface and a back surface opposite to the main surface, the heat radiation plate having through type slits penetrating from the main surface to the back surface; a semiconductor chip having a semiconductor element and a plurality of electrodes furnished on a principal plane, the semiconductor chip being fastened to the main surface of the heat radiation plate; a plurality of leads made of an inner lead and an outer lead each, tips of the inner leads being fixed to the heat radiation plate, the inner leads being connected electrically to electrodes of the semiconductor chip; and a molding member sealing the heat radiation plate, the semiconductor chip, and the inner leads; wherein the through type slits are laid out in a radial fashion radiating from outside a semiconductor chip mounting area of the heat radiation plate toward a region surrounded by the tips of the inner leads.
In the above semiconductor device, the heat radiation plate may be rectangular in shape and the through type slits may be fabricated to extend toward four corners of the heat radiation plate.
Further, according to the present invention, there is provided a semiconductor device comprising: a heat radiation plate including a main surface and a back surface opposite to the main surface, the heat radiation plate having through type slits penetrating from the main surface to the back surface; a semiconductor chip having a semiconductor element and a plurality of electrodes furnished on a principal plane, the semiconductor chip being fastened to the main surface of the heat radiation plate; a plurality of leads made of an inner lead and an outer lead each, tips of the inner leads being fixed to the heat radiation plate, the inner leads being connected electrically to electrodes of the semiconductor chip; and a molding member sealing the heat radiation plate, the semiconductor chip, and the inner leads; wherein the through type slits are laid out in a radial fashion radiating toward a region surrounded by the inner leads on the heat radiation plate.
In the above semiconductor device, the through type slits may be fabricated so that the back surface of the semiconductor chip is partially exposed.
In the above semiconductor device, the heat radiation plate may be rectangular in shape and the through type slits may be fabricated to extend toward four corners of the heat radiation plate.
As outlined above and according to the invention, the tips of the inner leads are fastened to the heat radiation plate. The structure eliminates the need for installing tab suspending leads supporting tabs that carry the semiconductor chip. The area that was conventionally allocated to the tab suspending leads is utilized for accommodating the inner leads. Given the same lead pitch, the structure allows the inner lead tips to be located closer to the semiconductor chip than before.
The inventive structure in which the inner lead tips are fastened to the heat radiation plate stabilizes bonding and prevents deformation of the inner leads.
According to the invention, the heat radiation plate has slits made therein in a radial direction forming heat propagation paths. The structure enhances protection against the reflow problem while minimizing a decline in heat radiation characteristic.
Also according to the invention, the inner lead tips are made thinner than before so as to improve accuracy in fabricating the tips. Fixing the inner lead tips to the heat radiation plate reinforces resistance to the deformation of the tips.
These and other objects, features and advantages of the invention will become more apparent upon a reading of the following description and appended drawings.